Whispering gallery resonators made out of lithium niobate allow for optical parametric oscillation and frequency comb generation employing the outstanding second-order nonlinear-optical properties of this material. An important knob to tune and control these processes is. e. g. https://www.roneverhart.com/Ibanez-GB10SEFMSRR-George-Benson-Semi-Hollow-Electric-Guitar-Sapphire-Red-p18747/